1. Field of the Invention
The present invention generally relates to printers, and more particularly to thermal printers having a thermal head serving as a printing head and a configuration allowing a platen roller to be mounted.
2. Description of the Related Art
In a printer of a stationary type POS terminal or a portable printer, a thermal head is provided as a printing head. Furthermore, one example of the portable printer is a clamshell type printer in which a thermal paper roll and its paper can be easily mounted. The clamshell type printer has a rotative openable lid that covers a thermal paper roll installing chamber in to which the thermal paper roll is installed. A platen roller is provided on a tip of this lid. When the lid is closed in a state and paper is pulled out from the thermal paper roll, the thermal paper roll is installed in the thermal paper roll installing chamber and the platen roller presses against a thermal head that is mounted onto a module of the printer. Thereby, the platen roller engages the module, so that the paper is set (sandwiched) between the platen roller and the thermal head and is ready for printing. In a case of exchanging the thermal paper roll for a new thermal paper roll after printing is continued for a certain period, the lid is rotatively opened. When the lid is rotatively opened, the platen roller disengages from the module. In other words, the clamshell type printer includes a platen roller that is engageable with its module.
In one example of a conventional thermal printer having an engageable platen roller, the thermal printer includes a sensor for determining (detecting) whether the platen roller is properly engaged with the module and another sensor for determining (detecting) whether there is paper sandwiched between the platen roller and the thermal head. In a case where the thermal printer is operated where the platen roller is not appropriately engaged or operated where there is no paper sandwiched between the platen roller and the thermal head, the thermal head may be damaged.
As another example of a conventional thermal printer having an engageable platen roller, there is a thermal printer whose number of sensors is reduced to a single sensor for reducing manufacturing cost. Accordingly, the thermal printer uses the single sensor to determine (detect) whether the platen roller is properly engaged and whether there is paper sandwiched between the platen roller and the thermal head.
FIG. 1 is a table showing the four states of a thermal printer having an engageable platen roller.
In the table shown in FIG. 1, <1> indicates a state where the platen roller is not properly engaged (disengaged) while there is no paper; <2> indicates a state where the platen roller is engaged while there is no paper; <3> indicates a state where the platen roller is not properly engaged while there is paper; and <4> indicates a state where the platen roller is engaged while there is paper.
FIGS. 2A-2E are schematic diagrams for describing a first example of a conventional thermal printer disclosed in Japanese Laid-Open Patent Application No. 2002-46321. As shown in FIG. 2A, the thermal printer includes a main body 1, a thermal head 2, a platen roller 3, an optical sensor 4, and a wire spring 5. The optical sensor 4 includes a light emitting part 4a and a light receiving part 4b. The optical sensor 4 is located at an area slightly separated from the area directly below the platen roller 3. The wire spring 5 has a U-shape when observed from above. The wire spring 5 is provided in the area where the optical sensor 4 is located. In FIGS. 2A-2E, reference numeral 10 indicates thermal paper.
As shown in FIG. 2B, when the platen roller 3 becomes engaged by closing a lid (not shown) where the paper 10 is drawn out along the thermal head 2, the paper 10 is sandwiched between the platen roller 3 and the thermal head 2, the platen roller 3 is pressed against the wire spring 5, and a part 10a of the paper 10 is positioned in the vicinity of the optical sensor 4 substantially directly below the platen roller 3. The light emitted from the light emitting part 4a is reflected from the paper 10 and directed to the light receiving part 4b. Thereby, the light receiving part 4b outputs high voltage. In other words, in the state <4> of FIG. 2B, the voltage output from the light receiving part 4b is high. In the state <2> where the platen roller 3 is engaged while there is no paper 10 as shown in FIG. 2C, the light emitted from the light emitting part 4a is not reflected and no voltage (0 voltage) is output from the light receiving part 4b. In the state <3> where the platen roller 3 is not properly engaged while there is paper 10 as shown in FIG. 2D, the light reflected to the light receiving part 4b is little due to the paper 10 being positioned substantially away from the optical sensor 4. Thus, low voltage is output from the light receiving part 4b. In the state <1> where the platen roller 3 is not properly engaged while there is no paper 10 as shown in FIG. 2E, no voltage (0 voltage) is output from the light receiving part 4b. In other words, as shown in FIGS. 2C-2E, the voltage output from the light receiving part 4b is either 0 voltage or low voltage in the states of <1>, <2>, and <3>.
Accordingly, detection can be achieved in the state of <4> of FIG. 2B.
FIGS. 3A-3E are schematic diagrams for describing a second example of a conventional thermal printer disclosed in Japanese Laid-Open Patent Application No. 2004-345264. As shown in FIG. 3A, the thermal printer includes a main body 21, a thermal head 22, a platen roller 23, and a load sensing type sensor 24. The load sensing type sensor 24 is located at an area slightly separated from the area directly below the platen roller 23.
As shown in FIG. 3B, when the platen roller 23 becomes engaged by closing a lid (not shown) where the paper 10 is drawn out along the thermal head 22, the paper 10 is sandwiched between the platen roller 23 and the thermal head 22, and a part 10a of the paper 10 is pressed against the load sensing type sensor 24. Thereby, the load sensing type sensor 24 is switched on. In other words, in the state <4> of FIG. 3B, the load sensing type sensor 24 is switched on.
In the state <2> where the platen roller 23 is engaged while there is no paper 10 as shown in FIG. 3C, no pressing force is applied against the load sensing type sensor 24. Thereby, the load sensing type sensor 24 remains off. In the state <3> where the platen roller 23 is not properly engaged while there is paper 10 as shown in FIG. 3D, a part 10a of the paper 10 is positioned away from the load sensing type sensor 24. Accordingly, no pressing force is applied against the load sensing sensor 24. Thereby, the load sensing type sensor 24 remains off. In the state <1> where the platen roller 23 is not properly engaged while there is no paper 10 as shown in FIG. 3E, no pressing force is applied against the load sensing sensor 24. Thereby, the load sensing type sensor 24 remains off. In other words, as shown in FIGS. 3C-3E, the load sensing type sensor 24 remains off in the states of <2>, <3>, and <1>.
Accordingly, detection can be achieved in the state of <4> of FIG. 3B.
However, in the first example of the conventional thermal printer shown in FIGS. 2A-2E, the optical sensor 4 is constantly exposed. In the state <3> shown in FIG. 2D where the paper 10 is pressed upward by the wire spring 5, the part 10a of the paper 10 is positioned away from the optical sensor 4 in a floating manner. However, in case where the paper 10 is excessively loosened (sagging), the part 10a of the paper 10 may be positioned in the vicinity of the optical sensor 4. Since the optical sensor 4 is exposed, the optical sensor 4 may erroneously determine that the current state is <4> even though the actual current state is <3>.
In a case where the optical sensor 4 erroneously determines that the current state is <4>, the paper 10 cannot be properly delivered even when the motor and the thermal head are driven by activating a printing operation. This leads to problems such as the thermal head 2 being damaged by heat.
Meanwhile, with the second example of the conventional thermal printer shown in FIGS. 3A-3E, in the state <3> shown in FIG. 3D, the load sensing type sensor 24 may be switched on even though the actual engagement of the platen roller 23 is insufficient in a case where the paper 10 is highly tensioned. Thus, the load sensing type sensor 24 may erroneously determine that the current state is <4>.
Likewise, in a case where the load sensing type sensor 24 erroneously determines that the current state is <4>, the paper 10 cannot be properly delivered even when the motor and the thermal head are driven by activating a printing operation. This leads to problems such as the thermal head 22 being damaged by heat.